Begin by charring the ginger, white onion, and shallot over an open flame or under the broiler until they are nicely charred.
In a frying pan over medium heat, toast the spices (Anise Seed, Fennel Seed, Coriander Seed, Cinnamon, Clove, Frctus Amoni) for about 1 to 2 minutes, or until they become fragrant and slightly darker in color.
Blanch bone marrow for 5 minutes before adding it to the broth to remove impurities and ensure a cleaner broth.
In your 5 qt pot or Instant Pot, add the charred ginger, onion, and shallot along with the Pho spices, bone marrow, rock sugar, salt, and water.
If using an Instant Pot, set it to high pressure and cook for 60 minutes. For the stovetop, bring the mixture to a boil, then reduce the heat and let it simmer for at least 2-3 hours, stirring occasionally.
After cooking the bones and spices in the Instant Pot, transfer them to a stove-top pot and bring them to a gentle simmer. Allow it to simmer uncovered for an additional hour, skimming off any foam or impurities that rise to the surface periodically. This extra simmering time will further enhance the flavor of the broth, resulting in a richer and more robust broth.